1. Field of the Invention
The present invention relates generally to modem communications, and, more particularly, to a method and apparatus for allocating tones to multiple end-users in a discrete multi-tone (DMT) modem communications system.
2. Description of the Related Art
The telecommunications industry has undergone explosive growth over the past several years. A significant contribution to this growth has been the high demand for modem communication services, such as the Internet, which extend beyond traditional voice communications. Conventional landline telephone networks, which offer xe2x80x9cPlain Old Telephone Servicexe2x80x9d (POTS), currently provide these modem services by transmitting data over a voice channel. The landline telephone network reaches nearly every household and business throughout the world, and has proven to be a relatively inexpensive medium for data transmission. Although dedicated data transmission networks also exist to provide these modern services, they are considerably more expensive to use. In addition, these data networks are not readily accessible to the general public, at least compared to the accessibility of the landline telephone network. Accordingly, a substantial portion of the public relies heavily on the landline telephone network as a cheaper alternative for providing these services.
With the increasing popularity of these modem services, however, the landline telephone network has rapidly become inefficient in handling these services due to limited bandwidth constraints. Currently, the landline network""s 56.6 kilobits per second (KBPS) data transfer rate is not fast enough to keep up with the increasing complexity of these services. In addition, projected demand for other services, such as video-on-demand, teleconferencing, interactive TV, etc. is likely to exacerbate the limited bandwidth problem.
To meet the demand for high-speed data communications, designers have sought innovative and cost-effective solutions that take advantage of the existing landline telephone network infrastructure. Of these solutions, the Digital Subscriber Line (DSL) technology, uses the existing landline network infrastructure of POTS for broadband communications, thus enabling an ordinary twisted pair to transmit video, television, and high-speed data.
DSL technology leaves the existing POTS service of the landline network undisturbed. Traditional analog voice band interfaces use the same frequency band (i.e., 0-4 kHz) for data transmission as for telephone service, thereby preventing concurrent voice and data use. Asymmetric Digital Subscriber Line (ADSL) technology, a popular version of DSL, operates at frequencies above the voice channels from 100 kHz to 1.1 MHz. Thus, a single ADSL line is capable of offering simultaneous channels for voice and data transmission. The ADSL standard is fully described in ANSI T1.413 Issue 2, entitled xe2x80x9cInterface Between Networks and Customer Installationxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interface, Rev. R4, dated Jun. 6, 1998, the entire contents of which is incorporated herein by reference.
ADSL systems utilize digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. ADSL systems provide a downstream data transfer rate from the ADSL Point-of-Presence (POP) to the subscriber location at speeds of about 6 Megabits per second (MBPS), which is more than one-hundred times faster than the conventional 56.6 KBPS transfer rate currently available through the landline network.
The technology employed in T1.413-type ADSL modems is discrete multi-tone (DMT). The standard defines 256 discrete tones, with each tone representing a carrier signal that can be modulated with a digital signal for transmitting data. The specific frequency for a given tone is 4.3125 kHz multiplied by the tone number. Tone 1 is reserved for the voice band and tones 2-7 are reserved for the guard bands. Data is not transmitted near the voice band to allow for simultaneous voice and data transmission on a single line. Thus, the guard bands aid in isolating the voice band from the ADSL data bands. Typically, a splitter may be used to isolate any voice band signal from the data tones. Tones 8-32 are used to transmit data upstream (i.e., from the user), and tones 33-256 are used to transmit data downstream (i.e., to the user). Alternatively, all of the data tones 8-256 may be used for downstream data transmission, and the upstream data present on tones 8-32 could be detected using an echo cancellation technique, as is well established in the art. Because a larger number of tones are used for downstream communication than for upstream communication, the transfer is said to be asymmetric.
ADSL technology significantly boosts the data transfer rate of the landline telephone network to levels at least commensurate with, if not exceeding, the transfer rates of present data networks. Assuming all 256 tones are free of impairments, the data transfer rate defined by the ADSL standard is about 6 MBPS downstream and 640 KBPS upstream, significantly dwarfing today""s data transfer rate standard on the landline telephone network.
Although ADSL technology dramatically increases the available bandwidth to a single user, the user generally does not require all of the additional bandwidth for his or her own purposes. As a result, a substantial portion of the additional bandwidth is not utilized. As such, the unused bandwidth is essentially a wasted resource.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the present invention, a method is provided for allocating a plurality of tone carriers in a communications system. The communications system includes a first modem and second modem coupled via a communications link. The first and second modems modulate data on the plurality of tone carriers for transmitting data over the communications link. The method includes determining a number of user devices that are to be coupled to the first modem. The plurality of tone carriers are divided into a number of subsets in accordance with a predetermined criterion, where the number of subsets corresponds to the number of user devices determined. Each subset of the plurality of tone carriers is allocated to a respective one of the determined number of user devices.
In another aspect of the present invention, an apparatus is provided that includes a first and second modem adapted to modulate data on a plurality of tone carriers. A communications link couples the first and second modem together and transfers data therebetween. The apparatus further includes at least two user devices adapted to couple to the first modem. The first modem divides the plurality of tone carriers into at least two subsets in accordance with a predetermined criterion, and allocates each subset of tone carriers to a respective one of the at least two user devices.